Chronic back pain is a cause for concern throughout the world and especially in the United States, affecting as many as 80% of all Americans at some point in their lives. Lower back pain can arise from any number of sources, including but not limited to conditions of the spinal vertebrae themselves, the intervertebral disks and the facet joints of the spine. Although the precise cause of back pain is still a matter of debate, it is recognized that nerves present in these structures contribute to the sensation and transmission of these pain signals. Some of the recent advances in the treatment of back pain, therefore, have focused on treating the nerves deemed to be contributing to the pain sensations.
A minimally invasive technique of delivering high frequency electrical current has been shown to relieve localized pain in many patients. The high frequency electrical current is typically delivered from a generator via one or more electrodes that are placed in a patient's body. Resistance to the high frequency electrical current at the tip of the electrode causes heating of adjacent tissue and when the temperature increases sufficiently, the tissue coagulates. The temperature that is sufficient to coagulate unmyelinated nerve structures is 45° C., at which point a lesion is formed and pain signals are blocked. This procedure is known as tissue denervation and it usually results in significant pain relief. Radio frequency (RF) denervation refers to tissue denervation using energy in the RF range. This technique has proven especially beneficial in the treatment of back pain and more specifically, lower back pain.
U.S. Pat. No. 6,736,835 B2, issued May 18, 2004, U.S. Pat. No. 5,571,147, issued Nov. 5, 1996 and PCT patent application WO 01/45579 A1, published Jun. 28, 2001, amongst others, disclose methods and devices for performing RF denervation of various tissues of the back, including spinal vertebrae, intervertebral disks and facet joints of the spine. In general, the procedure entails introduction of an electrosurgical device into the body, positioning the device proximate to the neural tissue to be treated and applying RF electrical energy in order to denervate the tissue.
More specifically, an electrosurgical device comprising a cannula having a hollow shaft and a removable stylet therein is inserted into a patient's body and positioned at a desired treatment site. The cannula typically comprises an elongate, insulated region, along with an electrically conductive and exposed distal tip. Once the distal tip of the cannula is in position, the stylet is withdrawn and the distal end of a probe capable of delivering high frequency electrical energy is inserted until the distal end of the probe is at least flush with the exposed distal tip of the cannula. The proximal end of the probe is connected to a signal generator capable of generating high frequency electrical current. Once the distal end of the probe is in position, energy is supplied by the generator via the probe to denervate the tissue proximate to the distal end of the probe.
Accurate placement of the cannula requires significant technical skill and is a crucial aspect of any denervation procedure. If the cannula, and through it the probe, is positioned incorrectly, the results for the patient can be disastrous, as the denervating energy may be applied to a region of tissue that should not be denervated.
In order to facilitate accurate localization of the cannula in tissue denervation procedures, X-ray fluoroscopy is used to observe the cannula and to help guide the cannula through the body. Contrast in fluoroscopic images is achieved by means of variation in the absorbance of x-rays amongst different materials. Materials that are relatively radiopaque, such as bones and most metals, appear darker on fluoroscopic images, in contrast to the relatively radiolucent soft tissues of the body. One limitation of the technique used currently for RF denervation is that the insulated shaft of the cannula is indistinguishable from the exposed distal tip of the cannula under X-ray fluoroscopy, due to the fact that the entire cannula, i.e. both the insulated as well as the exposed regions, is generally made up of a radiopaque substance. Therefore, precise localization of the conductive distal tip of the cannula is not possible as the entire cannula, comprising both the tip and the shaft, appears dark on the fluoroscopic image. Specific localization of the distal tip of the cannula is desirable as it is this region of the cannula that is electrically exposed and is therefore responsible for creating the lesion in the tissue.
In addition to fluoroscopy, two tests are typically conducted to confirm proximity to the target nerve and to confirm that the probe is not in proximity to other nerves prior to denervation. To assess proximity to the target nerve, an electrical stimulation is applied to the probe using a frequency that excites sensory nerves, typically 50 Hz with a current of up to 1 mA. A positive stimulation result reproduces the patient's pain, without producing other sensory responses in the lower extremity or buttocks. To confirm that the probe is not in proximity to an untargeted nerve, motor nerve stimulation is performed typically at a frequency of 2 Hz and a current of 3-5 mA. In this test, a lack of elicited muscle twitch in the lower limbs confirms that the probe is not at an undesired location near a spinal nerve. In the case of negative stimulation results, where there is a failure to reproduce the patient's pain or there is clear sensory or motor stimulation of the lower extremities, denervation is not performed. Rather, the probe is repositioned and proximity testing is repeated. Providing a manner of distinguishing the conductive distal tip of the cannula in fluoroscopic images may facilitate more accurate initial placement of the distal tip and avoid the requirement for probe repositioning.
Specifically with respect to facet joint denervation, positioning the cannula at the facet joint often requires the surgeon to steer or otherwise manipulate the trajectory of the device around a neural structure known as the sympathetic chain. The sympathetic chain refers to either of the pair of ganglionated longitudinal cords of the sympathetic nervous system of which one is situated on each side of the spinal column. Due to the proximity of the sympathetic chain which carries nerves that are critical to bodily function, facet joint denervation is a specific example of a procedure that may benefit from a manner of distinguishing the cannula upon insertion in the body. The clinical success rate of this procedure ranges from 9% (Lora & Long, 1976) to 83% (Ogsbury et al., 1977). The wide range of success rates is thought to be chiefly due to variability in positioning the electrode and the resulting lesion relative to the target nerve, even when using fluoroscopy and stimulation pulses. An improvement in technique and apparatus for positioning the cannula, and through it the electrical probe, proximate to the facet nerve may increase the success rate of this procedure and eliminate improper probe positioning as a reason for poor success.
The incorporation of radiopaque markers onto surgical devices has been used in the art to increase the visibility of such devices under x-ray fluoroscopy. While techniques vary for producing and incorporating radiopaque markers onto surgical devices, the general concept involves incorporating a material with high x-ray absorption onto a specified medical device. U.S. Pat. No. 5,429,597, issued Jul. 4, 1995, discloses a balloon catheter having a radiopaque distal tip composed of a polymer mixed with a radiopaque powder such as tungsten. U.S. Pat. No. 6,315,790 B1, issued Nov. 13, 2001, describes a catheter constructed with radiopaque polymer hubs wherein the hubs accomplish the dual functions of stent crimping and radiopaque marking. Another example of a catheter with a radiopaque marker is described in U.S. Pat. No. 5,759,174, issued Jun. 2, 1998. This catheter has a single external metal marker band used to identify the central portion of the stenosis once the delivery catheter is removed.
In all of the references noted above, radiopaque markers have been applied or attached to non-radiopaque devices, such as plastic or silicone-based catheters, and not to radiopaque devices such as metallic cannulae or needles. Furthermore, due to the incorporation of a radiopaque marker, in accordance with the current state of the art, onto a device for insertion into a patient's body, the force required to insert the device into the patient's body may be significantly increased, relative to the force required to insert a device lacking such a marker. This increased force, in turn, may result in unnecessary damage to bodily tissue during insertion of such a device into a patient's body. Thus, there is a need for an electrosurgical device that overcomes some or all of the limitations of the prior art.